This invention relates to a water soluble or water disintegrative core adapted for a precision casting for forming spaces in a cast product and also relates to a method for manufacturing a cast rotor of an induction motor provided with ventilation ducts by utilizing the core.
It is well known that solid casting preferably has been carried out by utilizing a core together with members constituting a cast rotor to form spaces in a cast product having a complicated configuration and, as a typical example of this fact, the inventors of this invention have proposed a method for manufacturing a cast rotor provided with ventilation ducts of an induction motor by utilizing a water soluble core, for example, as referred to in the Japanese Laid-open Patent Specification No. 70443/1980. Also well known is a method for manufacturing a cast rotor for a cage-type induction motor provided with conductors, together with short circuit rings and cooling blades, which are formed by pouring molten metal such as aluminium into slots formed by punching iron core plates usually made of silicon steel plates which were preliminarily laminated and clamped. A die casting or low pressure casting method is generally utilized for this purpose. Also known is a cast rotor adapted for an induction motor with a large capacity which is provided with ventilation ducts defined between blocks respectively made of laminated iron core plates for improving the cooling effect during the operation of the motor. In such a cast rotor, the blocks are connected only by conductors.
In a prior art (for example, Japanese Patent Publication No. 15402/1973), for forming ventilation ducts of a cast rotor there has been proposed a method comprising the steps of preliminarily forming duct spacers each having a width equal to that of the ventilation duct and provided with slots similar to those of an iron core plate by using a metal having a low melting point, laminating the spacers between the laminated core blocks, casting conductor metal thereinto, and heating and melting it to its melting point, if necessary, while rotating the rotor to remove molten metal.
However, with this method, the duct spacers made of a metal having a low melting point often melts and enters into the cast conductor when molten aluminum is poured and since workmen must work under a high temperature condition, working efficiency will be lowered. In addition, when the rotor rotates for effectively removing the spacers, the rotor has to be rotated at a low speed to prevent deformation of the conductors, so that it takes much time to remove the spacers. Moreover, this method requires an additional process such as heating process and it is troublesome to control the temperature of the core and the molten metal. For this reason, it may be required to coat a certain heat proof material on the surface near the slots.
Further, a method has been proposed for obviating defects of the methods described above, in which a water soluble core is utilized as a spacer instead of the spacer made of metal having a low melting point. According to this method, the core can be removed by dissolving or disintegrating it with water after the conductor metal has been cast, thus easily forming ventilation ducts.
However, such method as utilizes the water soluble core adapted for a cast rotor provided with ventilation ducts also has problems which are caused by the fact that materials for the water soluble core are not in satisfactory conditions indispensable to a precision casting.
Generally, it is required for the water soluble core or materials therefor to have the following characteristics:
(a) suitable moldability,
(b) excellent as cast strength (particularly, which is required in the method for manufacturing a cast rotor provided with ventilation ducts as described hereinbefore in which the core is used in combination with iron core plates, which are clamped for firmly combining them and in a pressure casting method, it is necessary for the core to have an as cast strength to withstand the pressure of the molten metal),
(c) prompt disintegration ability,
(d) no excess hygroscopicity and to be preserved in a usual dryer,
(e) proper dimensional precision, and
(f) smooth cast surface.
However, the water soluble core materials of the known types do not always have satisfactory characteristics that can meet the requirements described above.
For example, a mold product of a water soluble salt, for example, consisting of a large amount of sodium carbonate and small amount of barium carbonate (disclosed in the Japanese Patent Publication No. 15211/1975) has an excellent cast strength and smooth cast surface, but has a large thermal expansion coefficient and less dimensional precision in addition to a long time for the disintegration of the water soluble core and high cost for the use of a large amount of the molten salt. Moreover, a kneaded product consisting of alumina sand and water soluble carbonate such as sodium carbonate or potassium carbonate (for example, disclosed in the Japanese Patent Publication No. 28057/1975) has a good disintegration ability and moldability, but has less cast strength, so that such kneaded product can be used for gravity casting process, but cannot be used for low pressure casting process or die casting process and also cannot withstand a pressure at a time when laminated core blocks are clamped together with iron core plates and end plates.